With the rapid development of digitalization of information, digitalization in image processing is increasingly required. In digital cameras in particular, solid-state image pickup devices, such as Charge Coupled Devices (CCD) and Complementary Metal Oxide Semiconductor (CMOS) sensors, have been provided mainly on imaging planes instead of films.
In image pickup apparatuses including CCDs or CMOS sensors, an image of an object is optically taken by an optical system and is extracted by an image pickup device in a form of an electric signal. In one image pickup apparatus, light is regularly dispersed by a phase plate and is reconstructed by digital processing to achieve a large depth of field.
Devices like CCD and CMOS sensors that have image input functions sometimes read close-up still images, such as bar codes, together with desired images, such as landscape images. Techniques used for reading bar codes include an auto-focus technique in which focusing is performed by moving a lens towards and away from the bar code and a depth expansion technique in which the depth of field is increased by reducing the F-number in a camera so as to achieve fixed focus.
In some image pickup apparatuses, a Point Spread Function (PSF) obtained is constant when the above-described phase plate is placed in the optical system. The PSF describes the response of an imaging system to a point source or point object. The degree of spreading (blurring) of the point object is a measure for the quality of an imaging system. If the PSF varies, it can be difficult to obtain an image with a large depth of field by convolution using a kernel.
Therefore, setting single focus lens systems aside, in lens systems like zoom systems and autofocus (AF) systems, high precision is required in the optical design, thereby increasing costs accordingly. In one automatic exposure control system for a digital camera, filtering process using a transfer function is performed. More specifically, in known image pickup apparatuses, a suitable convolution operation cannot be performed and the optical system should be designed to eliminate aberrations, such as astigmatism, coma aberration, and zoom chromatic aberration that cause a displacement of a spot image at wide angle and telephoto positions. However, eliminating the aberrations can increase the complexity of the optical design, the number of design steps, the costs, and the lens size.
In a depth expansion technique, although a desired depth of field can be achieved at normal temperature, the back-focus position changes depending on high temperature or low temperature, causing the focal point to vary. Furthermore, a temperature change can possibly cause the lens to become loose or crack. In addition, if a plastic lens has high power, the performance thereof can vary significantly in response to a temperature change, making it difficult to achieve a satisfactory image quality even by performing a restoring process. Moreover, the depth of field varies depending on the surrounding environment.
Accordingly, there is a need for an image pickup apparatus which can reduce the change in the characteristics of lenses, reduce the degradation of the lens characteristics due to high or low temperature, and reduce the degradation in characteristics of back focusing change.